Semiconductors are widely used in integrated circuits for electronic devices such as computers and televisions. These integrated circuits typically combine many transistors on a single crystal silicon chip to perform complex functions and store data. Semiconductor and electronics manufacturers, as well as end users, desire integrated circuits which can accomplish more in less time in a smaller package while consuming less power. However, many of these desires are in opposition to each other. For instance, simply shrinking the feature size on a given circuit from 0.5 microns to 0.25 microns can increase power consumption by 30%. Likewise, doubling operational speed generally doubles power consumption. Miniaturization also generally results in increased capacitive coupling, or crosstalk, between conductors which carry signals across the chip. This effect both limits achievable speed and degrades the noise margin used to insure proper device operation.
One way to diminish power consumption and crosstalk effects is to decrease the dielectric constant of the insulator, or dielectric, which separates conductors. Probably the most common semiconductor dielectric is silicon dioxide, which has a dielectric constant of about 3.9. In contrast, air (including partial vacuum) has a dielectric constant of just over 1.0. Consequently, many capacitance-reducing schemes have been devised to at least partially replace solid dielectrics with air.
U.S. Pat. No. 4,987,101, issued to Kaanta et al., on Jan. 22, 1991, describes a method for fabricating gas (air) dielectrics, which comprises depositing a temporary layer of removable material between supports (such as conductors), covering this with a capping insulator layer, opening access holes in the cap, extracting the removable material through these access holes, then closing the access holes.
U.S. Pat. No. 5,103,288, issued to Sakamoto, on Apr. 7, 1992, describes a multilayered wiring structure which decreases capacitance by employing a porous dielectric. This structure is typically formed by depositing a mixture of an acidic oxide and a basic oxide to form a non-porous solid, heat treating to precipitate the basic oxide, and then dissolving out the basic oxide to form a porous solid. Dissolving all of the basic oxide out of such a structure may be problematic, because small pockets of the basic oxide may not be reached by the leaching agent. Furthermore, several of the elements described for use in this non-gel-based method (including sodium and lithium) are generally considered contaminants in the semiconductor industry, and as such are usually avoided in a production environment. Creating only extremely small pores (less than 10 nm) may be difficult using this method, yet this requirement will exist as submicron processes continue to scale towards a tenth of a micron and less.
Another method of forming porous dielectric films on semiconductor substrates (the term "substrate" is used loosely herein to include any layers formed prior to the conductor/insulator level of interest) is described in U.S. Pat. No. 4,652,467, issued to Brinker et al., on Mar. 24, 1987. This patent teaches a sol-gel technique for depositing porous films with controlled porosity and pore size (diameter), wherein a solution is deposited on a substrate, gelled, and then cross-linked and densified by removing the solvent through evaporation, thereby leaving a dry, porous dielectric. This method has as a primary objective the densification of the film, which teaches away from low dielectric constant applications. Dielectrics formed by this method are typically 15% to 50% porous, with a permanent film thickness reduction of at least 20% during drying. The higher porosities (e.g. 40%-50%) can only be achieved at pore sizes which are generally too large for such microcircuit applications. These materials are usually referred to as xerogels, although the final structure is not a gel, but an open-pored (the pores are generally interconnected, rather than being isolated cells) porous structure of a solid material.
As shown in the Brinker patent, semiconductor fabricators have used sol-gel techniques to produce dense thin films in semiconductors. The word sol-gel, however, does not describe a product but a reaction mechanism whereby a sol transforms into a gel. A sol is a colloidal suspension of solid particles in a liquid. One method of forming a sol is through hydrolysis and condensation reactions. These reactions cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles. Many monomers suitable for polymerization are metal alkoxides. For example, a tetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed in water by the reaction EQU Si(OEt).sub.4 +H.sub.2 O.fwdarw.HO--Si(OEt).sub.3 +EtOH
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer. TEOS which has been fully hydrolyzed becomes Si(OH).sub.4. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as EQU (OEt).sub.3 Si--OH+HO--Si(OH).sub.3.fwdarw.(OEt).sub.3 Si--O--Si(OH).sub.3 +H.sub.2 O
or EQU (OEt).sub.3 Si--OEt+HO--Si(OEt).sub.3.fwdarw.(OEt).sub.3 Si--O--Si(OEt).sub.3 +EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Si--O--Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation. Thus, additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers.
One theory is, that through continued reactions, one or more molecules in the sol may eventually reach macroscopic dimensions so that it/they form a network which extends substantially throughout the sol. At this point (called the gel point), the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, a gel can also be described as an open-pored solid structure enclosing a pore fluid. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
In a typical thin film xerogel process, an ungelled precursor sol may be applied to (e.g., spray coated, dip-coated, or spin-coated) a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried to form a dense film. The precursor sol often comprises a stock solution, a solvent, and a gelation catalyst. This catalyst typically modifies the pH of the precursor sol in order to speed gelation. In practice, such a thin film is subjected to rapid evaporation of volatile components. Thus, the deposition, gelation, and drying phases may take place simultaneously (at least to some degree) as the film collapses rapidly to a dense film. Drying by evaporation of the pore fluid produces extreme capillary pressure in the microscopic pores of the wet gel. This pressure typically causes many pores to collapse and reduces the gel volume as it dries, typically by an order of magnitude or more.
A dried gel that is formed by collapsing and densifying a wet gel during drying has been termed a xerogel. Typical thin film xerogel methods produce gels having limited porosity (Up to 60% with large pore sizes, but generally substantially less than 50% with pore sizes of interest). An aerogel is distinguishable from a xerogel primarily by largely avoiding pore collapse during drying of the wet gel.
U.S. Pat. No. 5,470,802, A Low Dielectric Constant Material For Electronics Applications, issued on Nov. 28, 1995 to Gnade, Cho and Smith describes a method for forming highly porous, finely pored (pore diameter of less than 80 nm and preferably of 2 nm to 25 nm), low dielectric constant (k less than 3.0 and preferably less than 2.0) dielectric films for use as semiconductor insulators. The U.S. '802 invention uses a surface modification agent to control densification and other shrinkage effects during drying, resulting in a substantially undensified, highly porous rigid structure which can be processed at atmospheric pressure. U.S. '802 teaches that the porous structure can be made hydrophobic (water repelling) and that the pores formed in the dielectric can be made small enough to allow this method to be used with device feature sizes in the 0.5 to 0.1 micron range, or even smaller. This results in a thin film that can be fabricated with almost any desired porosity (thin films with greater than 90% porosity have been demonstrated). Such films have been found to be desirable for a low dielectric constant insulation layer in microelectronic applications.
These techniques relate to fabricating dielectric (electrically nonconductive) materials, usually inorganic dielectrics. The inorganic porous dielectrics "aerogels" are nanoporous having average pore sizes less than 250 nanometers (preferably less than 50 nanometers and more preferably less than 10 nanometers and still more preferably less than 5 nanometers). Nanoporous dielectrics are of particular interest in advanced semiconductor manufacturing. The nanoporous inorganic dielectrics include the nanoporous metal oxides, particularly nanoporous silica.
Gnade et al.'s teachings include a subcritical drying method. That is, they dry the gelled film at one or more sub-critical pressures (from vacuum to near-critical) and preferably, at atmospheric pressure. Traditional aerogel processes typically replace the pore fluid with a drying fluid such as ethanol or CO.sub.2. The traditional processes then remove the drying fluid from a wet gel (dry) under supercritical pressure and temperature conditions. By removing the fluid in the supercritical region, vaporization of liquid does not take place. Instead, the fluid undergoes a constant change in density during the operation, changing from a compressed liquid to a superheated vapor with no distinguishable state boundary. This technique avoids the capillary pressure problem entirely, since no state change boundaries ever exist in the pores.